The invention relates to a method for determining from optical measurement a numerical measure of the deviation in the horizontal plane between the actual and the desired position of elongated parts of a machine, which parts are disposed rotatably adjacent to each other with their axis at an angle to the longitudinal axis of the machine, using an imaginary vertical reference plane substantially lying at a distance from the machine, which reference plane is determined by two base points lying a distance apart which set up an imaginary base line which is removed from and can be situated at an angle relative to the longitudinal axis of the machine, and by an optically reflecting device having a reflecting surface disposed in such a way that any cross-section thereof with a horizontal plane is a straight line which substantially a right angle with the imaginary reference plane, while an optical measuring instrument is disposed in line with the axis of the machine part concerned, or in line with an imaginary line substantially parallel to said axis, which measuring instrument is taken into a first position in which its optically operative side is directed towards the optically reflecting device, such that the line of sight of the measuring instrument is directed substantially parallel to the imaginary reference plane.
A method of this type is known, inter alia from American Patent Specification U.S. Pat. No. 4,003,133 in the name of the Applicant.
In this known method, in which the measuring instrument is a theodolite, the telescope of viewer thereof is directed from its first position towards the machine part concerned in such a way that the line of sight forms a right angle with the imaginary reference plane. Near one end of the machine part concerned, for example a roller, a measuring tab is placed substantially horizontally and at right angles to the axis of the machine part concerned, or to an imaginary line substantially parallel to said axis, such as, for example, a generating line of the roller surface. By using a plane-parallel plate situated in front of the telescope, the apparent distance between a particular measuring point on the measuring tab and the vertical cross-wire of the telescope is now determined in the direction parallel to the reference plane. The measuring tab is then placed near the other end of the machine part concerned, horizontally and at right angles to the axis, or to the imaginary line substantially parallel thereto, following which the apparent distance between the selected measuring point of the measuring tab and the vertical cross-wire of the telescope is determined again. The difference between the two measured distances is now a measure of the deviation in the horizontal plane between the actual position and the desired position of the machine part concerned. The measuring tab is in practice provided with different measuring points situated at known intervals, because in the case of a relatively large misalignment the distance to the initially selected measuring point can be too great to still be measured with a plane-parallel plate.
In order to be able to determine a measure of the misalignment of a machine part by the known method, a large number of operations still have to be carried out after the measuring instrument is placed in the position in which the line of sight of the telescope is directed substantially at right angles to the imaginary reference plane, and the main disadvantage can be seen in the fact that the measuring tab must be set up both at one end and at the other end of the machine part concerned. In practice, one of the ends of the parts of a machine to be aligned is often difficult or sometimes impossible to reach. If it is remembered that a machine can in practice sometimes be 200 m or even longer, it will be clear that much time and effort can be lost in positioning the measuring tab. For economic reasons it is desirable to take a machine out of operation for as short a time as possible for the alignment of the parts thereof.